I often throw out low offers on ebay, once in a while one gets accepted.
My latest item was a parts/repair 3458A. I was drawn to it for what is a silly reason - it is a very old unit. Serial # 170.
It has all the hallmarks of its age - dimming VFD, aged plastic and cracked/missing bits here and there. Just the way I like'em.
It powered up saying an ACAL ohms was required. It also had a sticker on the front from 2016 saying it was rejected as it failed ohms and current. Under that sticker was the cal sticker from the previous year where it appears to have passed.
At this point we opened the unit up and had a look under the hood. I was expecting to find rev 1 or 2 firmware but was surprised to see factory eproms with rev 9(current) firmware. The NVRAM's had also been changed ~ 2004. When I looked at the bottom of the unit I was greeted with an Agilent branded A3 board - so it had been replaced, likely at the same time the logic board was serviced. The AC board also has an Agilent sticker on it. So they either just added the sticker there or the AC board may have been swapped with a refurb as well. I was a little surprised to see the HI/LOW NVRAM's with a date code 4 years older than the CAL NVRAM. I assume they were all swapped at the same time and the HI/LOW units were just older stock. It looks like all work was done by Keysight(then Agilent).
At this point it was time for a little testing.
Running an ACAL ohms failed with an error indicating cal constant 98 was out of range. Running a normal resistance cal failed on the 10 mAh current range. Cal constant 98 is the stored value for the 10 mAh range which is measured through R211, a 9 ohm resistor on the A1 board. A quick measurement of R211 indicated it had drifted to just over 10 ohms. As a quick test a 100 ohm 1/4 watt resistor was jumpered in parallel to bring it back to ~ 9 ohms. An ohms CAL and ohms ACAL both passed with the quick and dirty fix. Could it be this simple? So far it seems like the resistor was the cause of the rejected cal. The challenge now is to find a properly rated 9 ohm resistor to replace it with. The part is still listed on the Keysight website but is obsolete and they don't have any.
Next was to dump the cal constants via GPIB(not likely useful, but it can't hurt) and then remove the logic board and desolder the NVRAM's. Sockets were added and battery free FM1808 FRAM's were used as replacements. They drop in directly to replace the HI/LOW(U121,U122) rams but an adapter is required to replace the CAL ram.
I also cleaned the unit, thankfully it didn't need to much. Mainly the dust filter and the fan. All of the older stickers were removed and I pushed out a dent that was in the top cover.
For now the 3458A is back together and running with its brothers. It's time for the real gamble to begin - how will the A3 perform. If it doesn't drift horribly the dream of an affordable 3458A is still alive. I'll report back with data in the coming weeks.
And until then does anyone have a suitable 9 ohm resistor recommendation, or perhaps have one from a parts board?
**A special thank you to ManateeMafia who helped get the unit to me, I have no access to my US address due to covid related border closures.**
**While troubleshooting I also did a few google searches for information, all of the results were from xDevs.com - so a shout out to TiN as well!**
Some pics:
Seriously aged LTZ1000A with the early internal LT part number:

Serial # and a curious rams due date:

Rev 9 firmware:

Original NVRAM's, CAL NVRAM 4 years newer for some reason we'll never know:

FRAM adapter and replacements installed:

Cal reject sticker that was on the front:

The quick resistor fix(temporary only of course):
